(1) Field of the Invention
This invention generally relates to sensors useful in electric control circuits and more particularly to a sensor that can measure temperature and magnetic field strength in an electric motor for facilitating motor control.
(2) Description of the Prior Art
Electric motors operate with diverse operating requirements, such as maximum efficiency, low noise, high torque and minimum physical size. Prior art motor control systems operate electric motors according to one or more of these or other requirements in response to various parameters, such as winding temperature. This particular parameter is important because it impacts the noise emanating from the motor. Moreover, as winding temperature increases winding resistance increases, so for a given voltage the magnetic field for the motor is reduced.
Stator temperature is another important parameter. Measuring stator operating temperature can provide an indication of operating efficiency. The simplest approach for monitoring stator temperature is to include a resistance temperature detector (RTD) or thermocouple (TC) in the windings. Resistance temperature detectors (RTD) and thermocouple sensors (TC) are inexpensive and easy to handle. However, their outputs are subject to electrical noise. Another approach is to use infrared (IR) thermometry. Sensors incorporating this technology provide a signal that has a significantly lower noise component. However, it has also been found that these systems are expensive and difficult to implement in a reliable manner often requiring constant monitoring of the sensor itself.
Another suggested approach for measuring temperature includes measuring temperature through emissivity measurements. U.S. Pat. No. 4,974,182 to Tank, for example, discloses one such method; U.S. Pat. No. 5,239,488 to Markham et al. discloses another implementation of an emissivity measurement. Although these sensors provide temperature measurements, their constructions are not conducive to inclusion in an electric motor, particularly at locations embedded in stator laminations which are optimal measurement locations. Moreover this apparatus provides only a temperature measurement.
Conventional sensors for measuring magnetic fields includes sense coils and Hall effect sensors. Sense coils are extremely reliable, require no external power and are easily understood. However, they are only capable of measuring alternating magnetic fields. They also have a large size and weight and the output signal has significant noise. Hall effect transistors, on the other hand, are comparatively small and lightweight. They are extremely reliable and measure both AC and DC fields. They require external power in the form a reference current in order to produce an output signal.
Some prior art references have proposed measuring both parameters. For example, U.S. Pat. No. 4,899,042 (1990) to Falk et al. discloses an integrated optic field sensor that includes an interferometer formed in a substrate. These integrated optical sensors measure electric, magnetic and temperature fields. The sensors are based upon stress-induced, refractive index changes in a first arm of a bridge. Electric and magnetic field sensors are also disclosed based upon evanescent field coupling between the field sensitive material and a first arm. While this patent discloses a system for measuring different parameters, a separate sensor is required for each parameter. Moreover, the method of sensing is to alter the path length.
Other devices have been proposed for using optical techniques for measuring magnetic fields. These include U.S. Pat. No. 4,433,291 (1984) to Yariv that discloses an optical fiber cable and magnetic field detector magnetostrictively reactive to the presence of an external magnetic field. A magnetostrictively responsive jacket disposed about the periphery of a fiber optic core responds to magnetic fields and strains the core effecting the light transmission through the core. An interferometer detects the changes. U.S. Pat. No. 4,591,786 (1986) to Koo et al. discloses a fiberoptic magnetic radiometer with variable magnetic biasing fields for measuring AC and DC magnetic field gradients. An optical interferometer includes magnetostrictive magnetic field sensing elements in each of the interferometer arms. U.S. Pat. No. 4,600,885 (1986) to Koo et al. discloses another fiberoptic magnetometer for detecting DC magnetic fields in which an AC magnetic field of a known frequency and constant amplitude is imposed such that the DC field introduces a detectable phase shift.
U.S. Pat. No. 4,622,460 (1986) to Failes et al. discloses a fiberoptic magnetic sensor with a thin walled cylindrical barrel having an elastically radially deformable mid-section. A single mode optical fiber is wound about the mid-section of the barrel and a magnetostrictive element is located within the center of the barrel. Field changes affect the length of the magnetostrictive element and consequently cause axial forces to be applied to the end of the barrel deforming the mid-portion and inducing a strain in the optical fiber from which the external magnetic field strength may be deduced.
U.S. Pat. No. 4,868,495 (1989) to Einzig et al. discloses a sensor with a single mode optical fiber for detecting electrical currents or magnetic fields. A current transformer utilizes a fiberoptic sensor and a phase modulated single mode fiberoptic interferometer such that a magnetostrictive element measures magnetic fields. This element comprises a tubular element having a slot along its length to allow the ingress and egress of a conductor, such as an electrical cable.
Each of the foregoing references discloses one of a number of diverse approaches for measuring either magnetic field or temperature. However, none of these references discloses a structure by which a single sensor provides information from which the values of both temperature and magnetic field strength can be deduced. Consequently, measurements of both require a duplication of the number of sensors. Such duplication increases volume and increases cost. Moreover many of the foregoing structures, particularly for measuring magnetic field, are not readily adapted for use in motor control circuits wherein the optimal position location of such sensors is between existing stator laminations in a motor.